ShipStation users create historical references as they create and modify data. Deleting users entirely would also delete the historical information. For this reason, users are instead 'deactivated' to maintain the integrity of their historical information. 'Deactivated' users can't log in, nor do they count against your plan's subscription limits. So, once a user is deactivated, it will function the exact same as having them deleted. And, don't worry, a deactivated user has no way to log back in or be added back to the ShipStation account other than an admin user on the account adding the username back.
Follow the directions below to deactivate or reactivate a user.
Head to your Account Settings (the wrench icon in the upper right).
Next, select Account and then User Management from the sidebar on the left.
Click on the Edit link for an existing user. In the "Information" sub-tab, look for the "Allow this user to login" setting.
Either click the checkbox away to deactivate the user, or add the checkmark to activate them.
When you're done, don't forget to Save!